Optical isolators are devices used in optical transmission systems to prevent back reflections in optical fibers. Back reflections can occur in fiber systems when light traveling in the system encounters an irregularity such as a change in refractive index between abutting materials or misalignment of fibers in the system. Back reflections result in reduced performance of the system and sometimes can adversely affect the transmission source, typically a laser.
Polarization dependent isolators utilize polarizers such as polarizing glass sheets to sandwich the Faraday rotator. In use, an isolator is disposed between two optical fibers or lenses such that light travels through a first polarizer, then through the Faraday rotator, and then through the second polarizer. In forward or pass mode operation, the incident light emitted from a light source such as a laser passes through the first polarizer. The remaining 50% of the light is then rotated 45° by the Faraday rotator before passing through a second polarizer offset from the first polarizer by 45°, preventing loss of signal. Polarized light emerges through the second polarizer. In reverse or blocking mode operation, reflected signal transmitted back through the isolator is polarized by the second polarizer before being rotated 45° by the direction independent Faraday rotator to a polarization mode 90° off from the first polarizer. Thus, no signal is transmitted back into the laser.
Another type of polarizer design utilizes a single polarizer, wherein in the pass mode, the emitted signal first passes through garnet and then through polarizer. The returned signal is polarized before passing through garnet, which rotates the return signal so that it is out of phase with emitted signal. This scheme promotes minimal laser interference. This design is less preferred than the isolator design described above which includes two polarizers, however, this single isolator design is less expensive than the dual polarizer isolator design.
Polarization independent isolators are preferred for applications where the incident signal is not already polarized. However, the emitted beam is not polarized. Polarization independent isolators include a Faraday rotator sandwiched between two beam splitters, which can be a birefringent material in wedge or plate form, or a prism with a thin film coating. In forward operation, incident light emitted from a laser is polarized by a first beam splitter into two distinct polarization modes. Each mode passes through a Faraday rotator and an optional half-wave plate, the latter correcting for the 45° rotation imparted by the Faraday rotator. The modes are then recombined by a second beam splitter into a non-polarized emission. In reverse or blocking mode operation, reflected light transmitted back through the isolator is separated into two distinct polarization modes by the second beam splitter. When each mode passes through the Faraday rotator and the optional half-wave plate, the signals are rotated 90° (due to the directional dependence of the half wave plate). When both rotated modes are recombined at the first beam splitter, the combined signal is transmitted 90° from the signal feed, thus preventing transmission (i.e., reflection) back into the laser.
Faraday rotators are typically made by surrounding a piece of garnet crystal with a magnet to apply magnetic field and make the crystal optically active. This type of garnet is referred to as non-latching. Another type of Faraday rotator utilizes a permanently magnetized or latching garnet that does not require an external magnet field.
The various component parts of both polarization dependent (Faraday rotator and polarizers) and polarization independent optical isolators (beam splitters and Faraday rotator) are typically held together by either mechanical assembly or by epoxy or polymeric adhesives. A limitation of mechanical assembly includes the introduction of optical signal loss due to air gaps that tend to exist between the surfaces of the adjacent parts, and the need to align and package individual components after dicing to final dimensions. An alternative method involves adhesive bulk assembly or lamination of large sheets of material followed by dicing, which avoids costs associated with packaging of individual, pre-diced parts. However, the adhesive assembly has the disadvantage of introducing optical loss when the epoxy or adhesive is in the optical path of the isolator. Another disadvantage of adhesive assembly is that when the isolator assembly encounters temperature variations, the epoxy or adhesive can fail due to CTE mismatches and/or temperature dependence of the adhesive's refractive index, causing delamination of the components. An additional disadvantage of adhesive assembly is that the epoxy may be susceptible to laser damage, causing optical loss, or, in some cases, catastrophic failure of the device in high power applications.
It would be desirable to provide an inexpensive and reliable method for bonding together the component parts of optical isolators. Furthermore, it would be desirable to achieve bonding of the isolator components without the use of adhesives or epoxy, while maintaining advantages of bulk assembly prior to dicing to final dimensions.